The human brain is like a long-term construction site – there’s always something else to be done


Synapses between nerve cells in the brain undergo constant remodeling, which is the basis of learning.

An LMU team has now traced the molecules that direct remodeling and shown that they circulate in the living cell like running sushi.

The human brain is like a long-term construction site – there’s always something else to be done.

This is certainly true of synapses, the functional links between nerve cells, which are constantly being strengthened, attenuated or demolished.

Indeed, this process termed synaptic plasticity is the basis of our ability to store and recall information – in other words, to learn.

The instructions for the synthesis of necessary components, which are encoded in molecules known as messenger RNAs (mRNAs), are delivered to the specific synapses that need them by a specialized transport system.

But how the blueprints reach their destinations is poorly understood.

Fluorescence micrograph of a hippocampal neuron growing in culture medium, showing the cell body (lower left) and dendrites. Synapses are imaged in violet and mRNA in green. Source: K. E. Bauer and M. A. Kiebler. Credit: LMU

In order to learn more about the underlying mechanisms, cell biologist Professor Michael Kiebler and his group at the LMU Biomedical Center have now followed the transport of individual mRNAs to specific synapses.

Their analysis shows that the same mRNA can be presented to potential addresses several times – a system which the researchers compare to running sushi, the use of an ‘endless’ conveyor belt to enable patrons to pick and choose from the delicacies on offer.

In order to serve the extensive network of synapses on a typically elongated process termed dendrite, the mRNAs must be transported from the nucleus in the cell body to the terminal branches at the end of the process.

To monitor this process, the LMU team used cell cultures derived from neurons isolated from the hippocampus of the rat, which serves as a model for the human hippocampus.

“We labelled specific mRNAs in living cells with a fluorescent dye, which enabled us to track their progress in real time,” Kiebler explains. “This approach permitted us to determine, for the first time, whether or not a given molecule is delivered directly to a particular synapse, and whether different mRNAs are handled differently in this respect.

In one case, we were able to follow how an mRNA entered one of the spine-like processes extended by a dendrite,” he says.

“Dendrites act as antennas that receive inputs from synapses on other cells.” The observations revealed that one and the same mRNA may repeatedly circulate back and forth between the cell body and the nerve processes – like sushi wending its way between the tables in a restaurant – until it finds a synapse that needs it.

Certain recognition sequences located in the segment of the mRNA that follows the stop codon (which marks the end of the protein-coding blueprint) serve as both the postage stamp and the address to direct the molecule to ensure that the molecule reaches the right region of the cell.

“We have also demonstrated that, if the postage stamp is left intact, transport from the cell body to the neural processes is more effective and the mRNA is brought closer to the synapse than when it has been removed,” says Kiebler.

In addition, RNA-binding proteins such as Staufen2 play an important role in the regulation of mRNA transport by this cellular sorting system.

Earlier studies had previously shown that Staufen2 is capable of binding several different mRNAs – so that the same mechanism can distribute distinct mRNAs.

In addition, the new report confirms early results which had suggested that uptake of the mRNA by the synapse depends on both the nature of the binding protein and the level of activity of the synapse.

Taken together, the new data provide further details on the mechanisms underlying the delivery of proteins to synapses, and will have an impact on future efforts to understand the molecular basis of synaptic plasticity in mammals.

Synapse formation requires the coordinated regulation of molecular and biochemical pathways in pre- and post-synaptic neurons.

This complex process involves multiple specializations in both pre- and post-synaptic compartments.

In pre-synaptic neurons, these specializations include the formation of active zones, vesicle clustering, the synthesis and release of neurotransmitters, and the synaptic enrichment of mitochondria (Lee and Peng, 2008Tang and Zucker, 1997Brodin et al., 1999Blaustein et al., 1978).

In post-synaptic neurons, post-synaptic densities and clustering of neurotransmitter receptors occur (Sheng and Kim, 2011Sheng, 2001).

Several studies have shown that retrograde signaling from the post-synaptic neuron modulates pre-synaptic signaling (Williams, 1996Sastry et al., 1988).

However, the mechanisms that maintain functional synaptic connections are poorly understood.

To address this, we focused on mechanisms that regulate the synaptic availability of mitochondria because they are critical for synapse formation and function (Guo et al., 2005Stowers etal.,2002Morris and Hollenbeck, 1993Misgeld and Schwarz, 2017Lin and Sheng, 2015MacAskill and Kittler, 2010Saxton and Hollenbeck, 2012Sheng and Cai, 2012Cai and Tammineni, 2017).

We considered the possibility that the post-synaptic neurons continuously send signals to the pre-synaptic neurons to ensure the availability of mitochondria at the pre-synapse.

Hence, we studied the active transport of mitochondria by kinesin-1 and dynein in the presence and absence of a functional synapse (Hirokawa et al., 1991Glater et al., 2006Schwarz, 2013Varadi et al., 2004).

To study transport, we used the advantages of identified presynaptic and post-synaptic neurons in the sea slug Aplysia californica.

Unlike vertebrate neurons that form autapses, Aplysia sensory neurons (SNs) and motor neurons (MNs) only form synapses between specific types of neurons (Kandel, 2001Bailey et al., 2015Glanzman, 2006Hawkins et al., 2006).

In the presence of L7MN, a target post-synaptic MN in the abdominal ganglia, SNs form glutamatergic synapses.

However, when SNs are cultured with L11MN, only fasciculation occurs (Lyles et al., 2006).

By culturing SNs with either L7MN or L11MN, questions about the specific effect of synapse formation can be addressed. Furthermore, because these neurons are large, SNs or MNs could be independently manipulated with high precision.

By comparing transport in SNs either grown alone or with a target or non-target MN, we find that synapse formation results in a persistent enhancement in bi-directional mitochondrial flux in pre-synaptic neurons.

This is coupled with a persistent alteration in the pre-synaptic transcriptome. Once the synapses are formed, the inhibition of signaling from the post-synaptic neuron does not alter persistent enhancement in transport. However, inhibition of pre-synaptic transcription or protein synthesis blocks the enhancement in transport.

More information: Karl E. Bauer et al. Live cell imaging reveals 3′-UTR dependent mRNA sorting to synapses, Nature Communications (2019). DOI: 10.1038/s41467-019-11123-x

Journal information: Nature Communications
Provided by Ludwig Maximilian University of Munich


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